These investigations are devoted to the development of non-invasive methods of accessing tissue structure and function. Two general techniques are being developed: nuclear magnetic resonance (NMR) and optical spectroscopy/imaging. Over the last year we have made the following developments in NMR technology: 1) Using the effects of deoxyhemoglobin and myoglobin on the magnetic relaxation properties of water in addition to the effects of vascular flow on tissue water relaxation, the effects of flow and blood oxygenation in the in vivo heart has been further evaluated. These studies demonstrated that the vascular volume of the myocardium is critical in the quantitation of perfusion or oxygenation using these relaxation terms. Methods for determining cardiac blood volume are under development. 2) Skeletal muscle and brain intracellular free Mg (Mgf) has been determined in a large population and compared to classical plasma determinations. No correlation between blood plasma levels and skeletal muscle Mgf was determined. This suggests that Mgf is not acutely regulated by plasma levels. 3) The NMR coil losses at 4T (170 MHz) in the human body have been characterized. Dielectric currents at 170 MHz were established as a major component to the NMR loss mechanisms at high fields. 4) A new planar gradient set has been constructed which provides gradient strengths up to 2.5 G/cm over the whole body. This is a key development for rapid cardiac imaging. 5) The magnetic field dependence of NMR relaxation pathways in human and model systems was determined which has provided unique insights into the mechanisms involved in this process as well as help make predictions for an optimal magnetic field for human NMR studies. In optical spectroscopy/imaging, a new system has been developed for the evaluation of in vivo cardiac optical signals using a rapid scanning spectrophotometer. This system is currently being used to evaluate the loading of specific calcium indicators into the myocardium, in vivo.